It is fundamental during the fabrication of semiconductor wafer to put one or more layers of conducting metal on the wafer surface and to include a patterning process that facilitates electrical connections between circuit components. The process is termed metallization. Although copper and gold are highly conductive metals and have been utilized in metallization processes, aluminum has evolved as the preferred conducting metal in the semiconductor fabrication process. A treatment of the metallization process can be found in a text book entitled: Microchip Fabrication, A Practical Guide to Semiconductor Processing., 3rd edition, by Peter Van Zant, McGraw-Hill, p.389-418. As discussed in the foregoing reference, aluminum suffers a problem called electromigration. The problem occurs when long, narrow leads of aluminum carry large currents over long distances, as is the situation in VLSI and ULSI circuits. Heat generated by the flowing current sets up a thermal gradient along the lead. The aluminum in the lead becomes mobile and diffuses along the direction of the current flow. The effect is to produce voids at one end and a surfeit of material at the other. In the extreme, the lead can become completely separated. The prior art teaches that electromigration may be minimized by depositing an alloy of aluminum and copper, or an alloy of aluminum and titanium on the silicon dioxide surface. Drawbacks for aluminum alloys, versus pure aluminum, are increased complexity for the deposition equipment and process, different etch rates, and an increase in film resistivity compared to the pure aluminum. The amount of the resistance increase varies with the alloy composition and heat treatments, but can be as much as 25 to 30 percent. Interdiffusion occurs when two materials are heated in contact with each other at temperatures much lower than their individual melting points. For example, if pure aluminum was put in contact with pure silicon (a pn junction), a great deal of silicon would be extracted from the small silicon contact and diffused into the aluminum, producing a crystallographically defined pit. If this process is permitted to proceed, the pit depth will be larger than junction depth, and these aluminum "spikes" short at the junction. As discussed in the textbook reference, both titanium-tungsten (TiW) and titanium nitride (TiN) layers are used to form barrier layers. TiW is sputter-deposited onto the wafer and into the open contacts before the aluminum or aluminum alloy deposition takes place. The TiW deposited on the field oxide is removed from the surface during the aluminum etch step. Sometimes a first layer of platinum silicide is formed on the exposed silicon before the TiW is deposited. According to the textbook reference, a titanium nitride layer may be formed by evaporation, sputtering, chemical vapor deposition (CVD), thermal nitridation in nitrogen or ammonia atmosphere. TiN, however, does require an underlying layer of titanium to promote adhesion with silicon substrates.
From a more in-depth point of view, electromigration lifetime performance is well known to be a function of trace width, aluminum grain size and orientation (texture) due to metal mobility in grain boundaries. This mobility is driven by the electron "wind," (momentum transfer from the electrons at high current density and temperature) and particularly to/from grain boundary "triple points." During electromigration, the aluminum in the narrow lead becomes mobile and diffuses along the direction of the electron flux and thermal gradients. Aluminum lines with a "bamboo" structure (grain size larger than line width and grain boundaries perpendicular to edge of line) have superior electromigration performance as compared to a randomly oriented small grain size. A single crystal metallization (no grain boundaries) would be the optimal metallization texture. Alternatively, if the underlying metal barrier forced the aluminum to be amorphous, (no long range order), there would be no grain boundaries to serve as high speed diffusion paths. The idea of using an amorphous metal barrier for improving the electromigration lifetime has been disclosed in a paper entitled: "Improvement of Electromigration Lifetime using Hyper-Textured Aluminum Formed on the Amorphous Tantalum-Aluminum Underlayer," by H. Toyoda et al. of Toshiba, IRPS (1994), pp. 178-184. Toyoda et al. discloses that an amorphous tantalum (Ta) barrier layer can produce superior texture to improve aluminum (Al) electromigration lifetime. However, the Toyoda et al. reference fails to disclose how the amorphous tantalum (Ta) barrier layer would be formed.
Accordingly, the primary object of the present invention is to provide a fabrication process for minimizing electromigration in the metal leads used in interconnecting circuit components in a semiconductor integrated circuit.
A particular object of the present invention is to provide a metallization fabrication process for producing an amorphous metal barrier such that the final interconnect metallization texture (Al or Cu) will be influenced by the underlying barrier layer and thus producing an interconnect metal texture that optimizes electromigration performance.
A related object is to provide a semiconductor circuit apparatus having an amorphous metal barrier layer and interconnecting metal leads formed in accordance with the foregoing objects.